Ball and seat valves for a wide variety of applications have been utilized for a number of years. The seat typically includes a bore adapted to receive and engage the ball, the bore constituting an inlet/outlet for a liquid medium. When the pressure behind the ball is higher than the pressure on the opposite side of the seat, the ball is forced into the bore thereby shutting off the flow of liquid through the valve. When the pressure behind the ball becomes lower than the pressure on the opposite side of the seat, the ball tends to disengage from the seat bore and the liquid can flow freely through the bore. Other factors may affect the circumstances under which the ball engages and seals the bore, such as gravity and buoyancy of the liquid medium.
Many industries have begun to realize a need for ball and seat valves that can withstand both higher pressures and more corrosive environments. For example, subsurface pumps utilized to extract oil from subterranean formations rely on ball and seat valves for efficient extraction of the oil. The operating pressure and environment are very severe, and the failure of a ball and seat valve will cause the pump system to completely cease operation, resulting in costly repair, downtime, and production loss.
Ball and seat valves utilized in the petroleum industry and other industries can be subjected to environments that include very corrosive chemicals such as hydrogen sulfide, carbon dioxide and salt water, for extended periods of time. Often to alleviate some of the problems associated with utilizing ball and seat valves in such environments, ball and seat valves have been manufactured from a variety of materials, typically metallic in nature. For example, stainless steel, nickel-copper alloys, bronze, and other materials such as cemented carbides have been utilized. However, the wear resistance of many of these materials has been found to be insufficient for many applications. The metallic materials tend to wear down and degrade after a period of time, particularly in highly corrosive environments. In the petroleum industry, the valves must often be replaced on a monthly basis.
In some applications, attempts have been made to utilize ceramic materials to alleviate some of these problems. Ceramics are known to be highly resistant to many corrosive environments, such as acids and salt solutions.
U.S. Pat. No. 4,795,133 by Berchem et al., issued Jan. 3, 1989, discloses a ball valve having a valve seat including two sintered ceramic seat rings and a ceramic valve ball. Berchem et al. disclose that the ball valve is especially useful for fluids containing solvents and/or abrasive solids. Similar ceramic ball and seat valves are disclosed by Berchem in U.S. Pat. No. 4,815,704, issued Mar. 28, 1989, and Berchem et al. in U.S. Pat. No. 4,771,803, issued Sep. 20, 1988.
The inherent brittleness of ceramics has severely limited their use in high pressure applications. With ductile metals, localized stresses that exceed the yield point are relieved by local plastic deformation that redistributes the stress into a wider area, preventing fracture.
Ceramics, however, have no such yield point and fail catastrophically when localized stresses exceed the material strength. Ceramics typically have a higher modulus of elasticity than metals which results in fracture at relatively small strains, which compounds the problem. The result is that when the stress on a ceramic seat exceeds the material strength, the seat will fail catastrophically and crack or break apart. The ball and seat valve is then completely useless and must be replaced, resulting in down-time and consequential economic loss.
Attempts have been made to alleviate this particular problem by placing the seat portion of the ball and seat valve in compression. See, for example, "Downhole Products," a sales brochure distributed by Coors Ceramics Company. By placing the seat in compression, tensile force applied by the striking ball would have to overcome the compressive forces to fracture the ceramic. The "Downhole Products" brochure discloses a tightly wrapped stainless steel ring around the seat to place the seat in a compressed state. However, this design is not completely satisfactory since the ceramic seat is still subject to fracturing during assembly and use. Fracture during use can occur primarily because the seating and sealing occurs on two flat, parallel surfaces.
There exists a need in many industries to utilize ball and seat valves in high pressure and highly corrosive environments. A ball and seat valve constructed from a corrosion resistant material, such as a ceramic, would be beneficial. It would be extremely beneficial if the ball and seat valve was designed so that the ceramic is able to withstand a maximum amount of stress without failing catastrophically. It would also be beneficial if this could be achieved without significantly altering the design of present ball and seat valves.